Electron beam current stabilizing device

ABSTRACT

Disclosed is an electron beam current stabilizing device comprising a sensing element (12) responsive to the beam current deviation from the predetermined value, a saw-tooth voltage shaper (14) connected to a high-voltage transformer (2) of an acceleration voltage source and providing periodic saw-tooth voltage, smoothly sloping portions thereof being shaped starting from the moment when the voltage at the high-voltage transformer (2) crosses zero, an adder (13) whose one input is connected to the output of the sensing element (12), and the other input, to the output of the shaper (14), a threshold element (15) connected to the output of the adder (13), a differentiator (16) connected to the output of the threshold element (15) to shape the electric driving pulses when the smoothly sloping portions of the saw-tooth voltage cross at the output of the adder (13) the threshold level of the threshold element (15), a light source connected to the output of the differentiator (16) to convert the electric driving pulses into the light pulses, and a photothyristor ( 11) controlled by light pulses and inserted into the primary winding of the heater transformer (5) supplied from one of the secondary windings of the high-voltage transformer (2).

FIELD OF THE INVENTION

The present invention relates to accelerator technique, and moreparticularly to electron beam current stabilizing devices.

BACKGROUND OF THE INVENTION

It is known that the magnitude of a charged particle beam current in theaccelerators designed for industrial application is a basic factordefining the radiation dose value within the accelerator irradiationfield. When the material is treated by a charged particle beam, forexample, by an electron beam, it acquires certain predeterminedproperties depending on the radiation dose. Unstability of the radiationdose results in deviation and spread of the properties of the irradiatedmaterial from the predetermined specifications. In order to provide thestability of the radiation dose it is very important to stabilize theelectron beam current.

The magnitude of the electron beam current is adjusted generally byalteration of the cathode emission current varying either the cathodeheater current or the intensity of the electric field around the cathodeof the accelerating tube.

Known in the art is an electron beam current stabilizing device (Cf. anarticle by Akoulov V. V. et al. "Promishlennie uskoriteli serii"Elektron" dlja radiatsionnoi himii", preprint of NIIEFA No. D-0198,Leningrad, 1974, p.II), comprising a ferroresonant voltage regulatorconnected to a secondary winding of an accelerator high-voltagetransformer, designed to supply the accelerating tube cathode heater,and an adjustable autotransformer connected to the output of theferroresonant regulator. Connected to the output of the autotransformeris a primary winding of the cathode heater transformer, theautotransformer being regulated by means of a reversible electric motorthe shaft of which is coupled through an insulating bar with a slidingcontact of the autotransformer. The reversible motor is actuated by anoperator controlling the magnitude of the electron beam current.

The aforementioned device provides considerably low beam currentstability because of, in the first place, low stability of ferroresonantvoltage regulators when alteration of the voltage in a supply networkoccurs, in the second place, the ambiquous relation between the cathodeheater voltage and the cathode temperature (the resistance of the heatercircuit and herewith the heater current may vary in the course ofoperation resulting in the alteration of the cathode temperature whichbrings, in its turn, again to the alteration of the heater circuitresistance etc.) whereas it is known that the cathode emission currentdepends exactly on the cathode temperature and, in the third place, theloss of emissive properties by the activated cathode due to the aging ofsaid cathode.

Besides, the aforementioned device fails to provide automatic adjustmentof the beam current and therefore a considerably large period of timemay pass from the moment of alteration of the beam current to the momentof effecting the control, during which the radiation dose will notcorrespond to a rated value thus leaving a part of the treated materialdevoid of the required property.

Known in the art is an electron beam current stabilizing device (C.f.Japanese Pat. No. 34514, published in 1974), comprising a thyristorcurrent stabilizer inserted in the cathode heater circuit. The beamcurrent is adjusted in this device, similar to the aforementioned one,by an operator actuating the current stabilizer through insulating barcoupled with the shaft of the reversible electric motor. The deviceaccording to this patent provides better stability of the beam currentsince the heater current and not the heater voltage define thetemperature of the cathode and consequenty the beam current. However,the stability of the beam current is still not sufficient due toparticipation of a man in the process of control.

Also known in the art is an electron beam current stabilizing device(Cf. U.S. Pat. No. 3,293,483 published in 1966), comprising aphotosensitive element connected to a cathode of an accelerating tube toadjust the beam current, and a light source controlling thisphotosensitive element. The photosensitive element is made as aphotoresistor. The cathode of the accelerating tube is connected througha secondary winding of the heater transformer and through thephotoresistor with a negative output of the acceleration voltage source.The negative output of the acceleration voltage source is also connectedto an accelerating tube modulator disposed near the cathode. The primarywinding of the heater transformer is connected generally to one of thesecondary windings of the high-voltage transformer of the accelerationvoltage source.

Means are provided in the device for regulation of the intensity, orbrightness, or the spectral composition of the light emanating from thelight source, which allow the resistance of the photoresistor to be setsuch that the potential difference between the cathode and the modulatorof the accelerating tube at a given particular value of the accelerationvoltage corresponds to the predetermined beam current.

The required beam current value is maintained automatically owing to thefact that when the beam current is deviated from the required value,voltage drops at the photoresistor and therewith the potentialdifference between the cathode and the modulator is altered, saidalteration of the potential difference being characterized by thereduction of said beam current deviation caused by said alteration.

The device according to this patent also fails to provide the sufficientaccuracy of stabilization of the electron beam current, which isattributed first of all to the low stability of the intensity andspectral composition of the light radiation as well as to the effect ofvarious interferences upon the transmitted analogue light signal.Unstability of the light radiation results in unstability of theresistance of the photoresistor. Since the light source is not insertedin the control circuit formed by the photoresistor, the cathode of theaccelerating tube and the modulator, the regulation error will beproportional to the variation in the parameters of the light source. Theaccuracy of the beam current stabilization is reduced also because of aconsiderable temperature unstability of the resistance of thephotoresistor approaching about 0.5-3% per a degree whereas thetemperature of the accelerator may vary in the course of its operationby as much as 30-40 degrees.

Furthermore the accuracy of stabilization in this device depends on thebeam current magnitude and, namely, decreases with the increase in thebeam current magnitude, which is explained by the fact that the greateris the portion of the acceleration voltage that drops at thephotoresistor, the higher is the accuracy of stabilization. When it isnecessary to increase the beam current the operator should decrease theresistance of the photoresistor through changing the light influence,thus decreasing the portion of the acceleration voltage of thephotoresistor, the stability of the beam current being consequentlylowered.

What is more, the increase in the beam current is accompanied also byadditional temperature unstability of the beam current attributed to theheat release at the photoresistor when the beam current flows across it.Thus, for example, to reach the accuracy of stabilization of the orderof about several percent, the voltage drop at the photoresistor shouldbe about 2-5% of the acceleration voltage. It does not bring toconsiderable heating of the photoresistor when the light beam current isequal, for example, to 100 microamperes, since the power released at thephotoresistor does not exceed several watts, while the power of aboutseveral kilowatts will be released on the photoresistor when the beamcurrent is equal, for example, to 100 milliamperes and the accelerationvoltage is equal, for example, to 500 kilovolts. Since heat removal fromthe high potential area of the accelerator presents difficulties, thephotoresistor will be strongly heated whereby its resistance willconsiderably vary resulting in the alteration of the voltage drop at thephotoresistor and therewith the beam current, i.e. in unstability of thebeam current.

The device according to the U.S. Pat. No. 3,293,483 cannot be also usedfor stabilization of heavy beam currents in modern high-poweraccelerators designed for industrial application because allphotoresistors known at present are capable to pass the currents notexceeding several milliamperes, the permissible voltages not exceedingthe tens of volts. In modern accelerators maximum beam currents reachhundreds of milliamperes and the potential of the modulator with respectto the cathode may reach several kilovolts. That is why in order tostabilize such beam currents in the aforesaid device at least severalhundreds of photoresistors would be required, connected in parallel andin series which, being of considerable dimensions, would occupy a largearea under the high potential where the space is extremely limited.

Besides, the device according to the U.S. Pat. No. 3,293,483 ischaracterized by considerable inertia because of photoresistor inertia,so that the time from the moment of setting the required resistancevalue of the photoresistor to the moment when the beam current reachesthe required value may be long enough, whereby a part of the materialbeing treated is irradiated by a reduced dose and thus becomes rejected.

SUMMARY OF THE INVENTION

The principal object of the present invention is to provide an electronbeam current stabilizing device wherein the accuracy of stabilization isincreased by eliminating the effect of the alterations of the parametersof the photosensitive element and the light source as well as of thebeam current magnitude upon the accuracy of stabilization.

With this principal object in view there is provided an electron beamcurrent stabilizing device for use with an accelerating tube having aheated cathode connected to a heater transformer supplied from one ofthe secondary windings of the high-voltage transformer of theacceleration voltage source, comprising a photosensitive elementconnected to the cathode of the accelerating tube to adjust the beamcurrent, and a light source to control the photosensitive element,wherein, according to the invention, the device further comprises asensing element responsive to the beam current deviation from thepredetermined value, a saw-tooth voltage shaper connected to thehigh-voltage transformer and providing the periodic saw-tooth voltage,shaping of the smoothly sloping portions of which starts at the momentswhen the voltage at the high-voltage transformer crosses zero, an adderwhose one input is connected to the output of the sensing elementresponsive to the beam current deviation from the predetermined valueand whose other input is connected to the output of the saw-toothvoltage shaper, a threshold element, connected to the output of theadder, and a differentiator connected to the output of the thresholdelement to shape the electric driving pulses when the smoothly slopingportions of the saw-tooth voltage at the output of the adder, cross thethreshold level of the threshold element, the light source beingconnected to the output of the differentiator to convert the electricdriving pulses into the light pulses, while the photosensitive elementis made as a photothyristor inserted in a primary winding circuit of theheater transformer.

The accuracy of stabilization of the beam current in the proposed deviceis increased due to the fact that the light source is hooked into thecontrol circuit, acting on the photosensitive element in accordance withthe magnitude of the beam current deviation from the predeterminedvalue, detected by the sensing element. The presence of the light sourcein the control circuit practically excludes the effect of unstability ofthe parameters thereof upon the accuracy of stabilization.

The use of the photothyristor as a photosensitive element allows toeliminate the effect of unstability of the resistance of thephotosensitive element upon the accuracy of the beam currentstabilization since the adjustment of the beam current, according to theinvention, is carried out not by means of alteration of the resistanceof the photosensitive element, but through alteration of the relation ofthe durations of time intervals corresponding to the cut off and firedstate of the photosensitive element.

The proposed device can provide practically any required accuracy ofstabilization irrespective of the magnitude of the beam current by meansof increasing the transmission factors of the elements constituting thecontrol circuit.

It is reasonable that the threshold level of the threshold element havesuch a value as to provide, in the absence of the beam current, shapingof the electric driving pulses by differentiator at the moments when thevoltage at the photothyristor rises to reach the value allowing, uponfiring of the photothyristor, the current equal to the holding currentthereof to flow therethrough.

This reduces the time from the moment of switching on of the acceleratorto the moment when the beam current reaches the required value.

It is also resonable that the device comprise a diode bridge inserted inthe primary winding circuit of the heater transformer and that thephotothyristor be inserted in a diagonal of the diode bridge, wherebythe supply of the heater transformer by the alternating current freefrom direct component is provided allowing thus to avoid constantmagnetization of the heater transformer core.

The device may comprise a Zener diode connected in series with thephotothyristor.

When the Zener diode is used the effect of the time required forregeneration of the photothyristor upon operation of the device iseliminated which allows to apply the device in the accelerators withhigher supply frequency.

The following description will be directed to the embodiments accordingto the present invention by referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an electron beam current stabilizingdevice, according to the invention;

FIG. 2 is another embodiment of connection of a photothyristor to aheater transformer in the device shown in FIG. 1, and

FIG. 3a to 3j are time diagrams illustrating operation of the deviceaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

An electron beam is shaped in an accelerating tube I (FIG. 1) whoseaccelerating field is produced by an acceleration voltage sourcecomprising a high-voltage transformer 2 having several secondarywindings of which the windings 3 are designed to generate theacceleration voltage, and the winding 4 is designed to supply a heatertransformer 5 connected to a cathode 6 of the accelerating tube I. Eachof the windings 3 is connected to a separate rectifier 7, all therectfiers 7 being connected in series to obtain the requiredacceleration voltage.

The cathode 6 of the accelerating tube I is connected electrically witha negative lead 8 of the acceleration voltage source. A positive lead 9of the acceleration voltage source connected electrically with the lastaccelerating electrode (not shown) of the accelerating tube I is earthedthrough a resistor 10 having low resistance and used for measuring theelectron beam current.

The beam current is adjusted with the help of a photosensitive elementrepresented, according to the invention, by a photothyristor II insertedin the primary winding circuit of the heater transformer 5. According tothe invention, the adjustment of the electron beam current is carriedout by alteration of the current of the heater cathode 6 effectedthrough varying the time intervals within the limit of each half-wave ofthe voltage on the high-voltage transformer 2 during which thephotothyristor II is maintained in a conducting state.

According to the invention, the device comprises further a sensingelement 12 responsive to the beam current deviation from thepredetermined value, an adder 13, a saw-tooth voltage shaper 14, athreshold element 15, a differentiator 16 and a light source to controlthe conductance of the photothyristor II. The sensing element 12responsive to the beam current deviation from the predetermined value isprovided with a comparison circuit 17 whose one input is connected tothe resistor 10 and the other, to the reference voltage source 18. Theinput of the saw-tooth voltage shaper is connected to the transformer 2,for example, to the low-potential winding 3 of the transformer 2, thoughit is possible, in principle, to connect the saw-tooth voltage shaper 14to a primary winding 19 of the transformer 2. However, the connection ofthe saw-tooth voltage shaper 14 across the secondary winding of thetransformer 2 is more preferable due to the fact that this eliminatesthe effect of phase shift between the primary and the secondary voltagesof the transformer 2 occurring when the load of the accelerator ischanged.

The saw-tooth voltage shaper 14 comprises, for example, a biphaserectifier 20, a threshold element 21, connected to the output of therectifier 20, and a saw-tooth voltage generator 22 started by thesignals from the output of the threshold element 21. The frequency ofthe saw-tooth voltage generated by the shaper 14 is twice as that of thevoltage at the windings of the transformer 2, the shaping of thesmoothly sloping portions of the saw-tooth voltage starting at themoments when the voltage at the windings of the transformer 2 crosseszero.

The output of the sensing element 12 is connected to one of the inputsof the adder 13 the other input of which is connected to the output ofthe saw-tooth voltage shaper 14, the adder 13 being made either ofresistors or on the base of an operational amplifier.

The output of the adder 13 is connected to the input of the thresholdelement 15 whose threshold level is chosen to be such as to provide inthe absence of the beam current the minimum voltage at the output of theadder 13 to be below the threshold level, and the maximum voltage to beabove the threshold level.

Connected to the output of the threshold element 15 is thedifferentiator 16 to shape the electric driving pulses whose shift withrespect to the moments when the voltage at the transformer 2 crosseszero is determined by the value and the sign of the beam currentdeviation from the predetermined value. As it will be shown below inconnection with description of the operation of the device, thesedriving pulses are shaped when the threshold level of the thresholdelement 15 is crossed by the smoothly sloping portions of the saw-toothvoltage generated at the output of the adder 13, corresponding to thesaw-tooth growth, i.e. from the leading edges of the output signal ofthe threshold element 15. The pulses shaped by the differentiator 16 atthe moments when the threshold level is crossed by the steep portions ofthe saw-tooth voltage at the output of the adder 13, corresponding tothe saw-tooth decay, i.e. from the trailing edges of the output signalof the threshold element 15, are not operative pulses and are not usedin the device.

Connected to the output of the differentiator 16 is the light sourcerepresented by light-emitting diodes 23 and shown in FIG. 1 as onelight-emitting diode. The light-emitting diodes 23 convert the electricdriving pulses at the output of the differentiator 16 into light pulsesfiring the photothyristor II, the number of the light-emitting diodes 23being defined by the intensity of the light flow required to fire thephotothyristor II. A flash-lamp or a laser can be also used as the lightsources instead of the light-emitting diodes 23.

Light pulses are transmitted from the light-emitting diodes 23 to thephotothyristor II through a light pipe 24, for example, a flexible lightpipe made of glassfibre or a rod made of organic glass.

According to one embodiment of the present invention the threshold levelof the threshold element 15 is such, that in the absence of the beamcurrent the electric driving pulses are shaped by the differentiator 16at the moments when the voltage at the photothyristor II rises to reachthe value at which the current on the photothyristor becomes equal tothe holding current thereof. In this case upon switching on of theaccelerator when the beam current is zero, the photothyristor II will beconducting practically during the whole half-period of the voltage atthe transformer 2, whereby full half-waves of sinusoidal voltage will beapplied to the heater transformer 5 and the cathode heater current willhave its maximum value exceeding the rated value required to maintainthe predetermined beam current. Hence the cathode 6 of the acceleratingtube I will be heated quicker than if it were heated by the rated heatercurrent, and the beam current will quicker reach the predeterminedvalue.

According to the embodiment of the present invention shown in FIG. 1,the photothyristor II is inserted directly between the secondary winding4 of the high-voltage transformer 2 and the primary winding of theheater transformer 5. In this case the photothyristor II will be firednot every half-period of sinusoidal voltage but every secondhalf-period. To provide the heater current flowing every half-period ofthe sinusoidal voltage, a diode 25, shown in the FIG. 1 by dotted lines,may be inserted in parallel and opposite to the direction of theconductance of the photothyristor II. In both cases, with the diode 25and without it, the heater transformer 5 will be supplied with thevoltage assymetrical with respect to the zero level. It is reasonable tomake the core of the transformer 5 split in order to prevent itssaturation when it is magnetized by direct component of the heatercurrent.

According to another embodiment of the present invention, as best shownin FIG. 2, the primary winding of the heater transformer 5 is connectedto the winding 4 of the transformer 2 through a diode bridge 26, thephotothyristor II being inserted in the diagonal of the diode bridge 26.The diode bridge 26 rectifies the voltage applied to the photothyristorII so that the latter will be fired each half-period of the voltage atthe transformer 2 and the heater current will have the shape symmetricalwith respect to the zero level.

In case when the supply frequency of the acceleration voltage source iscomparatively high, equal, for example, to 400 c.p.s. it may occur thatthe time interval from the moment of disconnecting of the photothyristorII as a result of the decrease of its current at the end of eachhalf-period of the sinusoidal voltage to the moment of its firing by thelight pulse at the beginning of the next half-period will be less thanthe time required for regeneration of the photothyristor II, this timebeing not long enough for the photothyristor II to be disconnected, thismaking it uncontrollable. To prevent this, a Zener diode 27 is insertedin the diagonal of the diode bridge 27 in series with the photothyristorII, the diode 27 having such a stabilization voltage that the voltage atthe photothyristor II is absent duting the period equal at least to theperiod of its regeneration, as a result of which the intervals whereinthe photothyristor II is cut off increase. The same effect may beobtained by inserting two oppositely poled Zener diodes in the circuitof the winding 4 of the transformer 2 in series with the diode bridge26. Since stabilization voltage of the Zener diode is chosen to be manytimes less than the maximum voltage of the heater supply, itsapplication does not affect much the power consumption.

The device according to the present invention operates as follows.

When the high-voltage transformer 2 (FIG. 1) is connected to thealternating current mains, voltage 28 (FIG. 3a) is produced in itssecondary windings 3, which is rectified by the rectifiers 7 and fed tothe cathode 6 of the accelerating tube I. The voltage produced on thesecondary winding 4 of the transformer 2 is fed through the diode bridge26 (FIG. 2) and the primary winding of the heater transformer 5 to thephotothyristor II. In the absence of the light pulses the photothyristorII is cut off and its voltage has the shape of a rectified sinusoid 29(FIG. 3b), the voltage in the primary winding of the heater transformer5 (FIG. 2) and therewith the cathode heater current being zero. Thus thecathode 6 is not heated and does not emit electrons even in the presenceof the acceleration voltage.

The voltage from the low-potential winding 3 (FIG. 1) of the transformer2 is applied to the shaper 14 at the output of which periodic saw-toothvoltage 30 is generated (FIG. 3c) changing in a cophasal way with therectified sinusoid 29 (FIG. 3b) and having smoothly sloping portionscorresponding to the growth of the saw teeth, and steep portionscorresponding to the decay of the saw teeth, the saw-tooth voltage beingeither lineary rising or lineary falling.

The comparison circuit 17 (FIG. 1) produces an error voltage 31 (FIG.3d) equal to the difference between the reference voltage of the source18 (FIG. 1) and the voltage produced on the resistor 10 by the loadcurrent of the acceleration voltage source equal practically to the beamcurrent. In the absence of the beam current the error voltage 31 hasmaximum value equal to the reference voltage as shown in the left partof FIG. 3d.

The error voltage 31 is added to the saw-tooth voltage 30 (FIG. 3c) inthe adder 13 (FIG. 1) and is applied to the input of the thresholdelement 15. The voltage at the output of the adder 13 is indicated bynumeral 32 and is shown in the FIG. 3e. The threshold element 15(FIG. 1) responds to the passage of the voltage 32 (FIG. 3e) over thethreshold level shown by a dotted line 33, and generates a singal in theform of rectangular pulses (FIG. 3f).

As it was mentioned above in order to accelerate the heating of thecathode 6 (FIG. 1) and hence to reduce the time required for the beamcurrent to reach the predetermined value, the threshold level of thethreshold element 15 is chosen such that the smoothly sloping portionsof the saw-tooth voltage 32 (FIG. 3e) from the output of the adder 13(FIG. 1) cross this threshold level at the beginning of each half-periodof the sinusoidal voltage 28 (FIG. 3a) when the voltage at thephotothyristor II (FIG. 2) reaches the value at which its current isequal to the holding current.

The differentiator 16 (FIG. I) transform the rectangular pulses 34 (FIG.3f) of the threshold element 15 (FIG. 1) into short pulses 35 (FIG. 3g)which are applied to the light-emitting diodes 23 (FIG. 1). Under theaction of the positive pulses 35 (FIG. 3g) formed as a result ofdifferentiation of the leading edges of the pulses 34 (FIG. 3f) of thethreshold element 15 (FIG. 1) light-emitting diodes 23 emit light pulses36 (FIG. 3h) the position of which, taken as a function of time, withineach half-period of the sinusoidal voltage 28 (FIG. 3a) bringsinformation about the beam current deviation from the predeterminedvalue. The negative pulses 35 (FIG. 3g) formed from the trailing edgesof the pulses 34 (FIG. 3f) of the threshold element 15 (FIG. 1) do notaffect the light-emitting diodes 23.

The light pulses 36 (FIG. 3h) fire the photothyristor II (FIG. 2) at thebeginning of each half-period of sinusoidal voltage, the photothyristorII remaining conducting practically during the whole half-period of thesinusoidal voltage and is cut off when the current in the primarywinding of the heater transformer 5 becomes equal to the holding currentof the photothyristor II. When the photothyristor II is fired thevoltage therein drops practically to zero and all the voltage generatedin the winding 4 of the transformer 2 is found to be applied to theprimary winding of the heater transformer 5 (the voltage on thephotothyristor II is indicated by the numeral 37 in FIG. 3i). As aresult, the heater current 38 (FIG. 3j) flows practically during thewhole half-period of the sinusoidal voltage so that the average value ofthis current is considerably higher than that of the rated heatercurrent required to maintain the predetermined current value. Thecathode 6 (FIG. 1) is intensively heated and when a certain temperatureis reached it starts to emit electrons shaped by the accelerating tube Iinto a beam.

The load current of the acceleration voltage source equal practically tothe beam current flows through the resistor 10 causing voltage dropthereat. As the load current increases the error voltage 31 (FIG. 3d)decreases reducing consequently the voltage 32 (FIG. 3e) at the outputof the adder 13 (FIG. 1) and the moments when this voltage passes overthe threshold level 33 (FIG. 3e) of the threshold element 15 (FIG. 1)are displaced to the right relative to the moments when the sinusoidalvoltage 28 (FIG. 3e) crosses zero. Consequently the light pulses 36(FIG. 3h) are also displaced to the right, whereby the photothyristor IIis fired later in each half-period of the sinusoidal voltage and theheater current 38 flows only during a part of the half-period of thesinusoidal voltage, as shown in the right side of FIG. 3j, i.e. theaverage value of the heater current decreases.

Displacement of the light pulses 36 (FIG. 3h) continues until theaverage value of the heater current 38 (FIG. 3j) reaches the valueproviding the predetermined beam current.

If in operation of the accelerator the beam current increases andexceeds the predetermined value, the error voltage 31 (FIG. 3d) producedby the sensing element 12 (FIG. 1) will change the sign, and the voltage32 (FIG. 3e) at the output of the adder 13 (FIG. 1) may decrease to thelevel when it does not cross the threshold level 33 (FIG. 3e) of thethreshold element 15 (FIG. 1). In this case light pulses 36 (FIG. 3h)will not be emitted, the photothyristor II (FIG. 2) remains cut off andthe current in the circuit of the heater cathode 6 ceases to flow untilthe beam current decreases down to the predetermined value.

When the photothyristor II is inserted in the primary winding circuit ofthe heater transformer 5 without the diode bridge 26, as shown in FIG.1, the device operates in a similar way except that the photothyristorII is fired during one half-wave in each period of the alternatingcurrent voltage.

Commercial applicability

The present invention may be widely used in the electron acceleratorsdesigned to irradiate different materials. By increasing the stabilityof the accelerator beam current the invention provides better stabilityof the radiation dose and consequently less spread in the properties ofthe irradiated material. Besides, due to reduction in time required toreach the predetermined value of the beam current the application of thepresent invention allows to increase the accelerator utilization factorand thereby to increase the efficiency of the accelerator for treatmentof the material with the electron beam.

We claim:
 1. An electron beam current stabilizing device for use with anaccelerating tube having a heater cathode connected to a heatertransformer supplied from one of the secondary windings of ahigh-voltage transformer of an acceleration voltage source, said devicecomprising a photosensitive element connected to the cathode of theaccelerating tube to adjust the beam current, and a light source tocontrol the photosensitive element, characterized in that it furthercomprises a sensing element (12) responsive to the beam currentdeviation from the predetermined value, a saw-tooth voltage shaper (14)connected to the high-voltage transformer (2) and providing periodicsaw-tooth voltage, smoothly sloping portions thereof being shapedstarting from the moment when the voltage at the high-voltagetransformer (2) crosses zero an adder (13) whose one input is connectedto the output of the sensing element (12) and whose other input isconnected to the output of the saw-tooth voltage shaper (14), athreshold element (15) connected to the output of the adder (13), and adifferentiator (16) connected to the output of the threshold element(15) to shape the electric driving pulses when the smoothly slopingportions of the saw-tooth voltage cross the threshold level of thethreshold element (15) at the output of the adder (13), the light sourcebeing connected to the output of the differentiator (16) to convert theelectric driving pulses into the light pulses while, the photosensitiveelement is made as a photothyristor (II) inserted in the primary windingcircuit of the heater transformer (5).
 2. A device as set forth in claim1, characterized in that the threshold level of the threshold element(15) has such a value, that in the absence of the beam current, theelectric driving pulses are shaped by the differentiator (16) at themoments when the voltage at the photothyristor (II) rises to reach thevalue allowing, upon firing of the photothyristor (II), the currentequal to the holding current thereof to flow therethrough.
 3. A deviceas set forth in claim 1 or 2, characterized in that it comprises a diodebridge (26) inserted in the primary winding circuit of the heatertransformer (5), the photothyristor (II) being inserted in the diagonalof the diode bridge (26).
 4. A device as set forth in claim 3,characterized in that it comprises a Zener diode (27) connected inseries with the photothyristor (II).